External AF motor for MF lenses

Why doesn't anyone make such a thing? It could involve these components:

an adapter, the size of other adapters available for various third-party MF mounts, which would communicate with the camera for autofocus commands

a motor which could rotate a clamp, which could be affixed to the MF lens's focus ring; the motor's external housing could be stabilized, perhaps by affixing it to the camera's tripod mount or the battery pack discussed below

an external battery pack to power the motor, which could be a fairly small plate-like device designed to affix to the bottom of the camera, via the tripod mount, and could offer a pass-through mount for a tripod/monopod underneath, and also could help hold the motor's housing steady